U.S. National Security Advisor Susan Rice (C) joins President Barack Obama (R) as he participates in a bilateral meeting with Kenya's President Uhuru Kenyatta (not pictured) at the State House in Nairobi July 25, 2015.

U.S. National Security Adviser Susan Rice said on Sunday that there are "relatively few" immediate security concerns stemming from Britain's vote to leave the European Union, but the United States will work to ensure continued U.S.-UK cooperation on counter-terrorism and other security issues.

Rice, in a forum at the Aspen Ideas Festival in Aspen, Colorado, said that the United States and Britain will "remain the closest partners and allies," while the need for NATO members to "stay latched up will be even greater."

"We will do all we can to ensure that the areas in which we are cooperating -- counter-terrorism, you name it, will remain solid," Rice said.

Asked what President Barack Obama first told British Prime Minister David Cameron when they spoke by telephone on Friday after the vote result was announced, Rice replied: "Bummer."

She added that the two leaders had talked through the potential outcomes of the vote on several occasions both in April when Obama visited Britain and urged a "remain" vote, and at a G7 summit in Japan in late May.

"In addition to a discussion about the consequences of the vote and conveying our respect for the will of the British people, it was also an opportunity for the president to underscore how much he appreciated David Cameron as a partner and as a friend," she said. "It's obviously a painful occasion when a partner that close experiences a loss that profound."

Obama and Cameron will work closely together during Cameron's remaining time in office and will also meet at an upcoming NATO summit in Warsaw in July.